| With the development of human society economy, people have paid more and more attention on the ecological environment protection and their own health. At present, the environmental pollution, in particular, air and water pollution are directly related to human survival and sustainable development of society economy. Titanium dioxide photocatalysis oxidation technology has been proved to be a low-cost and high efficiency method that can photodegrade the difficult degradable organic pollutants. Although TiO2 photocatalysis oxidation technology has already been partly applied in the field of environmental protection, it still exists several problems in the large-scale industrialized application, such as poor dispersion, difficult to recycle, the low utilization ratio of visible light. Diatomite, as a kind of natural porous mineral, has been used in the food and drug filtration, air and water purification, heat preservation materials, and industrial catalyst carrier. And because it has rich pore structure, so diatomite is a good adsorber for environmental pollutants. But the disorption and secondary pollution limit its practical application. By loading TiO2 nanoparticles successfully on diatomite, the TiO2/diatomite composite material can not only overcome the poor dispersibility and difficult recycling of TiO2 nanoparticles but also overcome the disadvantages of diatomite itself. This study used organic titanium as titanium source to prepare nano-TiO2/diatomite composite photocatalysts via sol-gel method and calcination. The rare earth metal ions, like cerium and lanthanium, and transition metal ion vanadium were used to improve the visible light photoactivity of nano-TiO2/diatomite composites. At the same time, further study of preparation, material structure and the relations between them and the photocatalytic performance of nano-Ti O2/diatomite composites were carried out. The main work of this paper and the main achievements are as follows:(1) The preparation technique and the performance of nano-TiO2/diatomite composite photocatalytic materialsUsing the purified Jilin Changbai diatomite as carrier of TiO2 nanoparticles. Using organic titanium as the titanium source, nano-TiO2/diatomite composites were prepared via sol-gel method. The optimized preparation technique of nano-TiO2/diatomite photocatalysts is as follows: Pulp density of 50 g/L(anhydrous ethanol dosage is 20 m L), v(H2O)/v(HAc)=6(acetic acid dosage is 1.0 m L, deionized water dosage is 6.0 m L), tetrabutyl titanate dosage of 1.5 m L, heat treatment condition of 750℃ for 2 h, and heating rate of 2.5℃/min. Scanning electron microscope(SEM), transmission electron microscope(TEM), X-ray diffraction(XRD), Fourier infrared spectrometer(FTIR), UV-vis diffuse reflection spectrometer(UV-vis DRS), X-ray photoelectron spectrometer(XPS), and other characterization methods were used to characterize the structure and properties of nano-TiO2/diatomite composite material. Combining with the experiments of photocatalytic degradation of rhodamine B, the effects of the v(H2O)/v(HAc) value, dosage of tetrabutyl titanate and the calcination temperature on the photocatalytic performance of composites were analyzed and discussed, respectively. The results showed that the photocatalytic activity of nano-TiO2/diatomite composites increased first and then decreased with the increasing v(H2O)/v(HAc) value, dosage of tetrabutyl titanate and the calcination temperature. In addition, the pure TiO2 prepared with the same conditions was characterized as well. Compared with the pure TiO2 particles, the nano-TiO2/diatomite composites was of anatase-rutile mixed crystal phase, the anatase phase accounted for 90%, the average grain size was 19.1 nm. Under 1 h UV light illumination, the removal of rhodamine B reached 92.6%. It confirmed that the diatomite as carrier binded TiO2 crystals by Si—O—Ti bonds, which retarded phase transformation and grain growth of TiO2. The TiO2 grain size, crystal structure and phase composition, and the distribution on the diatom surface determine the photocatalytic performance of the nano-TiO2/diatomite photocatalysts. This result has significant theoritical and practical values for the industriallization development of nano-TiO2/diatomite composites preparation.(2) Nano-TiO2/diatomite composite materials with rare earth metal ions doping and modification mechanismBased on the optimized preparation conditions the nano-TiO2/diatomite composites were doped by cerium or lanthanium. And the relationship between doping concentration and photocatalytic performance was studied, as well as the doping mechanism. The TiO2 crystal phase and grain size, the existence of doping ions and the optical properties of composites were characterized by XRD, TEM and XPS, UV-vis DRS and PL spectrometer analysis. The results showed that the Ce4+/Ce3+ failed to enter TiO2 crystal lattice, but existed on the surface of TiO2 grains as oxides, binding with TiO2 via Ce—O—Ti bond. The new Ce4 f level decreased the transition distance of TiO2 valence band electron, resulting in the effectively reduction of bandgap and the enhanced visible light activity. When the doping concentration of Ce was 1.5 mol%, the bandgap of composites reached the minimum, 2.75 eV. At the same time, Ce4+ ions are more effective than the surface adsorption oxygen in the capture of the photo-generated electrons, so it could improve the separation efficiency of photo-generated electron and hole. The photodegradation of rhodamine B over 1.5%-Ce/TD was 72.03% under the stimulated solar light irradiation by 5 hs.Compared with Ce doping, the La dopants also could not enter TiO2 crystal lattice. The results showed that the La formed La—O—Ti bond and La2O3 oxide, leading to retard TiO2 grain growth and crystal phase transformation. However, La doped led to a blue shift of absorption band edge, and to increase recombination rate of photo-generated electron and hole. This indicates that La doping is not an appropriate modification method for the nano-TiO2/diatomite composites.(3) Nano-TiO2/diatomite composite materials with transition metal ions doping and modification mechanismBased on the optimized preparation conditions the nano-TiO2/diatomite composites were doped by vanadium. And the relationship between doping concentration and photocatalytic performance was studied, as well as the doping mechanism. The TiO2 crystal phase and grain size, the existence of vanadium ions and the optical properties of composites were characterized by XRD, TEM and XPS, UV-vis DRS and PL spectrometer analysis. The results showed that part of the V4+ ions entered the TiO2 lattice and substituted for Ti4+, when doping concentration was 1.0 mol%, leading to lattice distortion and inhibiting the growth of TiO2 grain. While the others were V5+ ions that failed to enter the TiO2 crystal lattice, but attached to the surface of TiO2 grain in the form of V2O5. V4+ ions introduced V3 d impurity level under the bottom of TiO2 conduction band, which effectively reduced the bandgap of V-TiO2/diatomite composites. The bandgap of composites exhibited the minimum of 2.95 eV when the V doping concentration was 0.5 mol%. V5+ ions could act as the capture of photo-generated electron, effectively improving the separation efficiency. The photodegradation of rhodamine B over 1.0%-V/TD was 93.09% under the stimulated solar light irradiation by 5 hs. |
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